The present disclosure relates generally to electrophotographic imaging members. More specifically, the present disclosure relates to imaging members having enhanced durability. In particular, the imaging members comprise a crack-deterring backing layer on the side of the substrate opposite that of the imaging layers.
In the art of electrophotography, an imaging member or plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation, for example light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles, for example from a developer composition, on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving substrate such as paper. This imaging process may be repeated many times with reusable photosensitive members.
Imaging members are usually multilayered photoreceptors that comprise a substrate support, an electrically conductive layer, an optional hole-blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, and an optional protective or overcoat layer(s). For some multilayered flexible photoreceptor belts, an anti-curl layer is employed on the reverse side of the substrate support, opposite to the side carrying the electrically active layers, to achieve the desired photoreceptor flatness.
Imaging members are generally exposed to repetitive cycling, for example by the rollers of a printing machine. This cycling leads to a gradual deterioration in the mechanical and electrical characteristics of the electrically active (i.e. photoconductive) layers. In particular, repetitive cycling can cause cracks to form in the outermost exposed layer, i.e. the charge transport layer or the overcoat layer. Cracks are problematic because they can manifest themselves as print-out defects which adversely affect copy quality. Charge transport layer cracking thus reduces the service life of the copier or printer.
In addition, the operating environment exposes the imaging member to several conditions, which can decrease its service life. The imaging member is exposed to several airborne chemical contaminants. Typical chemical contaminants include solvent vapors, environment airborne pollutants, and corona species emitted by machine charging subsystems such as ozone. It is also subjected to constant mechanical interactions against various subsystems. These mechanical interactions include abrasive contact with cleaning and/or spot blades, exposure to toner particles, carrier beads, toner image receiving substrates, etc. In particular, these mechanical interactions can scratch the outermost exposed layers. Again, these scratches impact copy quality and service life.
High crack resistance and high scratch resistance are therefore desirable attributes. Overcoat layers help increase both crack resistance and scratch resistance. However, the material properties that are favored to increase one many times undermines the other. For example, a low Young's modulus is desired for increased crack resistance, but a high Young's modulus is desired for increased scratch resistance. The Young's modulus should not exceed 20 GPa and is usually no higher than 5 GPa for flexible imaging members (i.e. belts). The yield strength should no exceed 100 MPa and is usually no higher than 50 MPa. In addition, the overcoat layer should not interfere with the electrical properties of the imaging member. Consequently, the overcoat layer should provide high crack resistance and high scratch resistance, with low interference with the electrical properties of the underlying layers.
Generally, a compromise between these goals is required. Thus, rather than being excellent in all properties, an overcoat is usually excellent in one property and only good in others. It is desired to provide a backing layer that reduces the requirements of an overcoat layer. This would allow the overcoat layer to be tailored to be enhanced in one or more of the desired properties.